1. Field of the invention
This invention relates to an inclined magnetic field generation coil and a magnetic field generator which are used in a magnetic resonance imager (MRI).
2. Description of the related art
MRI is an imaging apparatus using a magnetic resonance phenomenon and is being extensively used for purposes of medical diagnosis and the like. Conventionally, normal conducting electromagnets, superconducting electromagnets and the like have been used for the generation of a magnetic field for MRI. However, as a result of the recent development of high-performance rare-earth permanent magnets, it has become a major trend to use rare-earth permanent magnets (hereinafter referred to briefly as “permanent magnets”) for the generation of a magnetic field for MRI, for example, in MRIs having a low magnetic field of 0.5 T or less.
A conventional magnetic field generator for MRI and the pole pieces and other components used therein are described below with reference to FIG. 10. FIG. 10 is a cross-sectional view of a magnetic field generator for MRI as viewed from the side. In FIG. 10, plate-like yokes 101 and 102 are supported by a columnar yoke 103. On this pair of plate-like yokes 101 and 102, generally discoidal permanent magnets 104 and 105 selected from the group consisting of Nd—Fe—B, Sm—Co and Sm—N—Fe magnets are disposed so as to face each other. Moreover, pole pieces 106 and 107 having a circular base are attached to the opposed faces of permanent magnets 104 and 105, respectively.
Permanent magnets 104 and 105 are each magnetized along the thickness, keeping the direction of magnetization from being antiparallel.
On the other hand, peripheral projections 106b and 107b are provided along the periphery of pole pieces 106 and 107 (i.e., the periphery of their bases), respectively. These peripheral projections 106b and 107b serve to produce a magnetic field having a uniform strength substantially in the center of the space between pole pieces 106 and 107, and these projections have an approximately constant height. Bases 106a, 107a and peripheral projections 106b , 107b are formed, for example, of a soft magnetic material laminated onto a soft iron material such as low-carbon steel or pure iron.
In the recesses on the gap side of pole pieces 106 and 107, a pair of inclined magnetic field generators 108 and 109 are disposed in order to produce an inclined magnetic field on the gap side of the opposed pole pieces. The main purpose of this inclined magnetic field generator is to act on the uniform magnetic field space on the gap side of the pole pieces and thereby disturb the uniformity of the magnetic field linearly by intention. Then, if NMR signals including the nonuniform magnetic field are received, spatial information can be added during image formation from the signals. In an ordinary magnetic field generator for MRI, three inclined magnetic field coils arranged orthogonally so as to coincide with the X-axis, Y-axis and Z-axis of a three-dimensional coordinate system. The technology of inclined magnetic field coils is described, for example, in Japanese Patent Provisional Publication No. 63-65848/'88.
Now, referring to FIG. 11 which is an enlarged cross-sectional view of the pole piece regions shown in FIG. 10, the construction of the pole piece regions on the gap side of the inclined magnetic field generator is described in greater detail. For example, on the gap side of inclined magnetic field generators 108 and 109, RF wave transmitters (also called transmission coils) 110 and 111, magnetic field regulation mechanisms 112 and 113, a subject carrying table 114, and the like are usually mounted. Thus, the effective gap length of an actual magnetic field generator for MRI is expressed by the following equation (1)Lg=L0−(2×Ts+Tt)  (1)or by the following equation (2)Lg=L1−{2×(Ts+Tb)+Tt}  (2)The symbols in these equations are defined as follows.
Lg: Effective gap length.
L0: Gap-side distance between peripheral projections 106b and 107b. 
L1: Gap-side distance between inclined magnetic field coils 108 and 109.
Ts: Thickness of transmission coil
Tt: Thickness of subject carrying table
Tb: Thickness of magnetic field regulation mechanism.
These equations (1) and (2) indicate that the space available for the imaging of a subject is gradually narrowed by the mounting of various component parts.
The aforesaid magnetic field regulation mechanisms 112 and 113 disposed on the gap side of the inclined magnetic field coils serves as a tool for making a final magnetic field adjustment during installation in a hospital or the like. For example, each of them comprises a resin plate having about 10 to 200 magnetic field regulating holes 117 formed therein, as illustrated in FIG. 12. By inserting any desired magnet pieces or magnetic material pieces (e.g., iron pieces) into the regulating holes, the magnetic field uniformity on the gap side of the pole pieces can be enhanced. However, since the number of holes is large and a high machining accuracy is required, the magnetic field regulation mechanisms are regarded as one of the cost-raising factors among various component parts of the magnetic field generator for MRI.